He was probably seeing what response there would be from ATC to an incorrect alt selection.
Some ATC sites allow the controller to see the selected altitude in the ADS-B data but many do not from what I've read here on other threads.
I've been quizzed by a Chinese controller about a metric RVSM altitude in the window but perhaps he was checking the QNE mode C readout after level off rather than selected altitude from ADS-B.
Would the Germanwings A320 send out ADS-B data about the selected altitude and would the controllers be able to see it in this case? Anybody know?